The present invention relates generally to wire bonding apparatus and methods used in the manufacture of integrated circuit packages. More specifically, the present invention relates to methods and apparatus used to wire bond an integrated circuit package component such as a die that utilizes heat sensitive contacts such as copper contacts.
The semiconductor industry has been moving continuously toward smaller and faster semiconductor devices with higher transistor density and increasing numbers of input/output connections. These trends have led to the desire to replace the more common aluminum metallization layers within semiconductor devices with copper containing metallization layers. As is well known to those skilled in the art, this use of copper rather than aluminum provides substantial benefits such as higher speed and improved conductivity, and reduces the problems caused by the inductance and capacitance of the features formed in the metallization layer.
Although the use of copper metallization layers in semiconductor devices improves the performance of the device, these devices are difficult to package using conventional wire bonding techniques. This difficulty is mainly due to the fact that, compared to conventional aluminum metallization layers, copper metallization layers oxidize very quickly when exposed to oxygen in the air. This is especially the case when copper is exposed to air at an elevated temperature as is typically required during conventional wire bonding processes.
Wire bonding remains the most common chip interconnecting method for fine pitch semiconductor devices. Gold or aluminum wire is commonly used to connect an input/output terminal pad of the semiconductor die to a lead of a interconnecting substrate such as a leadframe. Typically, a ball bond is used to connect a first end of a bonding wire to the input/output terminal pad while a wedge bond, also called a stitch bond, is used to connect the bonding wire to the lead of the leadframe. Conventional wire bonding apparatus utilize a capillary that is driven by the wire bonding apparatus to form the bonding wire. The capillary is also used for both the ball bonding and the stitch bonding processes.
For illustrative purposes, a conventional wire bonding apparatus 100 that is used to wire bond a conventional integrated circuit die to the leads on a leadframe will be described with reference to FIGS. 1-3. As shown in FIG. 1, wire bonding apparatus 100 includes a heater block 102 for supporting leadframe 104 and an integrated circuit die 106. Die 106 includes a plurality of input/output terminal pads 108 that are to be electrically connected to a plurality of associated leads 110 on leadframe 104 using an array of bonding wires 112 that are formed during the wire bonding process. As shown in FIG. 1, bonding wires 112 are attached to associated input/output terminal pads 108 using a ball bond 114. The other end of each bonding wire 112 is attached to an associated lead 110 using a stitch bond 116.
Leadframe 104 is held in position using a window clamp 118 which mechanically clamps leadframe 104 against heater block 102. Heater block 102 includes a vacuum port 120 having a vacuum cup 122 that is used to securely hold die 106 in place during the wire bonding process. Heater block 102 also includes heating elements 124 that run through heater block 102. As described in more detail hereinafter, heating elements 124 are used to heat heater block 102, and therefore leadframe 104 and die 106, to a desired temperature to aid in the wire bonding process.
Wire bonding apparatus 100 further includes a capillary 126 that has a longitudinally extending wire feed bore 128 that is used to extrude bonding wires 112. To assist in the wire bonding process, wire bonding apparatus 100 is capable of placing a desired downward force on capillary 126 during the process of forming ball bond 114 and stitch bond 116. Apparatus 100 also typically is capable of delivering ultrasonic energy to the bonding region through capillary 126 to enhance the bonding process. As mentioned above, heater block 102 is heated to a desired temperature, typically a temperature greater than about 150 degrees centigrade, to aid in the bonding process.
FIGS. 2A-C briefly illustrate the basic steps involved in forming a bonding wire using capillary 126. FIG. 2A is a close up partial cross sectional view of capillary 126. As illustrated in FIG. 2A, a free air ball 130 of a bonding wire material such as gold or aluminum is formed at a distal end 132 of capillary 126. This is typically accomplished using an electronic flame off mechanism 134 which applies energy to distal end 132 of capillary 126. Once free air ball 130 is formed, it is used to form ball bond 114 on input/output terminal pad 108. FIG. 2B illustrates a close up vertical cross sectional view of capillary 126 as it is being used to form ball bond 114. As mentioned above, a combination of heat, pressure from capillary 126, and ultrasonic energy are used to attach ball bond 114 to pad 108.
Once ball bond 114 is attached to pad 108, bonding wire 112 is extruded through wire feed bore 128. With an appropriate length of bonding wire extruded, bonding wire 112 is stitch bonded to an associated lead 110 on leadframe 104. FIG. 2C illustrates a close up vertical cross sectional view of capillary 126 as it is being used to form stitch bond 116 on lead 110. Again, a combination of heat, pressure, and ultrasonic energy are used to make stitch bond 116.
Traditionally, the above described wire bonding process requires a substantial amount of heat in order to insure good quality bonds at ball bond 114 and stitch bond 116. Typically temperatures of greater than 150 degrees centigrade are required. When utilizing heat sensitive metallization layers such as copper or copper alloys to form the input/output terminal pads 108 on die 106, these relatively high temperatures accelerate the oxidation of the copper pads which can prevent the formation of a good reliable bond between ball bond 114 and pad 108. Therefore, to reduce this problem, it is desirable to develop methods and apparatus capable of forming ball bonds at a reduced temperature.
One approach to reducing the amount of heat required to form ball bonds is described in detail in two copending U.S. Patent Applications, which applications are incorporated herein by reference. These two U.S. patent applications are: Ser. No. 08/784,271, entitled METHOD AND APPARATUS FOR FINE PITCH WIRE BONDING, attorney docket number NS3463/NSC1P087; and Ser. No. 08/890,354, entitled ENCAPSULATED BALL BONDING APPARATUS AND METHOD, attorney docket number NS3681/NSC1P098.
FIG. 3 illustrates a close up partial cross sectional view of a capillary 140 similar to capillary 126 described above. However, capillary 140 includes a specific tip configuration such as that described in detail in the above referenced patent applications. This specific tip configuration, which is referred to as encapsulated ball bonding, allows capillary 140 to be used to form a ball bond 144 without requiring as much heat as is typically required to form reliable ball bonds. In fact, as will be described in more detail hereinafter, applicants have found that the encapsulated ball bonding methods and apparatus may be used to form high quality ball bonds without requiring any substantial heating of the contacts to which the ball bonds are to be attached
To summarize the above referenced patent applications, the encapsulated ball bonding apparatus and methods will be briefly described with reference to FIG. 3. As shown in FIG. 3, capillary 140 has a longitudinally extending wire feed bore 141 that is used to extrude bonding wires 112. Capillary 140 also has a cavity 142 formed into its tip. Cavity 142 has a cavity diameter 143 and is shaped and sized such that cavity 142 substantially encapsulates and is capable of molding a ball bond 144. Among other things, this configuration allows capillary of this design to deliver ultrasonic energy to ball bonds more effectively than conventional tip configurations. Because of this, reliable ball bonds may be formed without requiring any substantial heating of the input/output terminal pads to which the ball bonds are to be attached.
In a specific example, a number of experiments have been successfully performed using a model 3006FPX wire bonding machine available from ESEC. Using such a machine and a capillary having a 1.0 mil gold bonding wire and a cavity diameter of about 1.6 mil, bonding times on the order of 15 milliseconds at 14% power worked well. In the experiment, the bonding temperature was about 120 degrees centigrade and the bonding force exerted downward on the ball bond during the bonding process was about 100 mN. Of course, the settings used for various applications may be widely varied.
Because the above described encapsulated ball bonding approach does not require heating of the contact to which the ball bond is to be attached, this approach is well suited to ball bonding on heat sensitive metallization layers such as cooper or copper alloys. By eliminating the need to heat the contact to which the ball bond is to be attached, the amount of oxidation that occurs during the overall bonding process may be substantially reduced. However, although the encapsulated ball bonding approach works well to produce a ball bond without requiring any substantial heating, this is not the case for forming good quality stitch bonds. In the case of stitch bonds, the encapsulated ball bonding approach still appears to require the heating of the lead to which the stitch bond is to be attached in order to provide reliable connections.
To date, wire bonding is one of the most cost effective and popular packaging methods of packaging integrated circuit die. Because of the popularity of this packaging approach, conventional wire bonding equipment is readily available. Accordingly, it is desirable to provide methods and apparatus that will allow conventional wire bonding apparatus to be modified so that integrated circuit packages using heat sensitive metallization layers such as copper or copper alloy may be reliably assembled. The present invention provides improved wire bonding methods and wire bonding apparatus that substantially reduce the oxidation problem associated with the use of higher performance heat sensitive metallization layers such as copper and copper alloys. This allows integrated circuit packages using the higher performance materials to be reliably assembled using cost effective wire bonding methods and apparatus in accordance with the invention.